At present many transactions are conducted, especially in stores and restaurants, in electronic form using a memory card, known as a bank card, incorporating a microprocessor in a semiconductor chip. The memory card is inserted into the card reader of a portable payment terminal which can exchange transactional data with a stationary base terminal which is capable of being in communication with a payment center, usually via the public switched telephone network. The payment center processes the transactions transmitted to it and can grant or refuse a requested authorization.
The prior art includes systems for exchanging data between portable payment terminals and stationary base terminals in which a portable terminal and the associated base terminal each includes a central unit and a radio transmit-receive module. The central unit of the portable terminal is able to generate binary messages to be transmitted to the base terminal, with the messages including the amount of the transaction, the identity of the customer and the identity of the trader, for example. The messages are transmitted to the base terminal by radio via the radio transmit-receive module of the portable terminal. The base terminal can transmit binary messages to the portable terminal using its radio transmit-receive module, for example, to authorize or to cancel a transaction.
The above prior art data exchange systems operate in the following manner, but it must be understood that the roles of the portable terminal and the base terminal are totally symmetrical. During transmission, the central unit and the radio transmit-receive module of whichever terminal is transmitting operate simultaneously in the sense that the data received from the central unit is transmitted to the receiving terminal by the transmit-receive module as and when it arrives. Switching in the central unit during the generation of binary messages causes electromagnetic interference at the transmit-receive module, in particular at the antenna, thereby producing noise in the transmit band. However, in this case such interference has only a slight effect on the signal being transmitted given that the signal is amplified to a relatively high level.
During reception, on the other hand, operation of the central unit of the receiving terminal to read the binary messages as they are received by the radio transmit-receive module causes a relatively higher level of interference because the signal received can be at a low level, in particular when there is a relatively great distance between the portable terminal and the base terminal. The noise generated by the central unit of the receiving terminal then renders the signal from the transmitting terminal virtually unusable, and this limits the range of the system.